As the implementation of high-speed signal transmission in a mounting circuit board progresses, impedance matching of a multi-layer wiring board becomes more important. In a transmission line for electrical signals, in a case where the output-side voltage is constant, by configuring the output impedance of the circuit and the input impedance of a reception-side circuit to be the same, power acquired at the reception-side circuit becomes a maximum. Accordingly, in order to efficiently transmit an electrical signal, such impedances need to be the same (that is, to match each other).
In a wiring board, in order to match the impedances, it is important that resistance, reactance, and conductance (capacitance) of the wiring board should be manufactured to have designated expectation values.
However, in the case of the multi-layer wiring board, in a stage in which the manufacturing thereof is completed, it is difficult to measure and check the conductance (hereinafter, also referred to as capacitance) that is a constituent element in units of layers configuring the multi-layer wiring board.
For example, as disclosed in U.S. Pat. No. 3,603,850, in inspecting a multi-layer wiring board, an inspection method has been known in which the quality of an electric wiring is determined by measuring capacitance generated between the wiring to be inspected and an electrode opposing the wiring.
FIG. 4 is a schematic cross-sectional diagram that illustrates a conventional method of inspecting a multi-layer wiring board.
In the conventional inspection method illustrated in FIG. 4, a wiring board 1101 to be inspected includes an electric wiring 1120 that is configured by a lower layer wiring 1121 and an upper layer wiring 1122 inside the board and has a multi-layer wiring structure. The opposing electrode 1102 is arranged to face the wiring board 1101. The size of the opposing electrode 1102 covers the entire wiring board 1101 and is electrically connected to a capacitance meter 1110 in a part thereof.
In addition, the capacitance meter 1110 is electrically connected to a terminal 1123 or a terminal 1124 disposed on the electric wiring 1120 of the wiring board 1101.
In the inspection method illustrated in FIG. 4, the quality of the electric wiring 1120 is determined by measuring capacitance between the electric wiring 1120 and the opposing electrode 1102 using the capacitance meter 1110, wherein the electric wiring 1120 is arranged to face the opposing electrode 1102.
More specifically, in a case where a short-circuit defect does not occur in the electric wiring 1120, the capacitance measured by the capacitance meter 1110 is a combined capacitance that is acquired by combining capacitance C100 between the lower layer wiring 1121 configuring the electric wiring 1120 and the opposing electrode 1102 and capacitance C200 between the upper layer wiring 1122 configuring the electric wiring 1120 and the opposing electrode 1102. Since the capacitance is the same as a designated expectation value, the electric wiring 1120 can be determined as a non-defective product.
On the other hand, for example, when there is a short-circuit defect between the lower layer wiring 1121 and the upper layer wiring 1122, in a case where the capacitance meter 1110 is connected to the terminal 1123, only the capacitance C100 between the lower layer wiring 1121 and the opposing electrode 1102 is measured. In addition, in a case where the capacitance meter 1110 is connected to the terminal 1124, only the capacitance C200 between the upper layer wiring 1122 and the opposing electrode 1102 is measured. In any of the cases, the measured capacitance is smaller than the combined capacitance of the capacitance C100 and the capacitance C200 that is an expectation value, and accordingly, the occurrence of a short-circuit defect in the electric wiring 1120 can be detected.
In this way, in the conventional inspection method, the measurement of capacitance using the opposing electrode is effective in a case where the quality of the electric wiring of the multi-layer wiring board is to be determined.
However, the conventional method of inspecting a wiring board through the measurement of capacitance is a method in which such combined capacitance is measured instead of measuring the capacitance in units of layers of the multi-layer wiring structure. In other words, the conventional inspection method is not for the purpose of measuring capacitance in units of layers of the electric wiring, and, in the conventional inspection method, while capacitance is measured using the opposing electrode, the capacitance in units of layers cannot be measured.
As described above, in a wiring board, in order to match the impedances, it is important that the capacitance value is produced according to a designated value. Further, it is a requirement to check whether the capacitance value is produced according to the designated value or not, and if required, the result of the checking process is to be fed back to the production process. However, the method of inspecting a wiring board and the like that are conventionally known cannot be applied to such a checking process.
In other words, in a case where capacitance of each layer unit of a wiring board having a multi-layer wiring structure is to be checked, a conventional inspection apparatus for a wiring board or a conventional inspection method in which capacitance is measured using an opposing electrode cannot be used.
As described above, in the case of the wiring board having a multi-layer wiring structure, in a stage in which the board is completed, it is difficult to measure capacitance that is a constituent element thereof in units of layers configuring the wiring board using a conventional method. Accordingly, in the wiring board, in order to match the impedances, the wiring board is required to be manufactured to have a designed capacitance value. However, after the wiring board is manufactured, it is difficult to check whether or not the capacitance has an expected value. As a result, in a case where the capacitance of the wiring board deviates from the expected value, there is a problem in that it is difficult to improve the manufacturing condition and the like by having such a deviation to be fed back to the manufacturing process of the wiring board.
Thus, a method of inspecting a wiring board having a multi-layer wiring structure in which capacitance that is a constituent element is measured in units of layers configuring the wiring board in a stage in which the wiring board is completed and an inspection apparatus for a wiring board that realizes the method have been requested. In addition, a technique for enabling the manufacturing of a wiring board having an expected capacitance value by having a measurement result to be fed back to the manufacturing process of the wiring board has been requested.
The present invention has been contrived in consideration of such problems.
In other words, an object of the present invention is to provide an inspection apparatus for a wiring board that measures capacitance in units of layers of the wiring board having a multi-layer wiring structure.
In addition, another object of the present invention is to provide a method of inspecting a wiring board for measuring capacitance in units of layers of the wiring board having a multi-layer wiring structure.
The other objects and advantages of the present invention become clear in the following description.